Thirding the Tstat. The difference between a tstat and a rheostat is the the rheostat has no way to measure the heat out put. Plug your heat pad into a rheostat/dimmer switch and you can manually turn down the power out put to the heat pad, ensuring it won't get to it's maximum temps, but then it stays at what ever you set it too regardless of any other factors such as room temps. Like calmolly said, if you want to keep that hot spot stable, you'll end up have to constantly fiddle with it. Thermostat has and extra feature: a prob that measures temps and sends back to the unit what the heat pad is currently doing. Then the thermostat will increase or decrease the power output, automatically adjusting for factors to make sure the temps are stable. Depending on your model, tstats can also come with a variety of sanity saving features; things like auto shut offs if a malfunction happens, warning indicators that can sound alarms if it gets too hot or too cold, and lots of other stuff. It's worth the peace of mind to spend the extra $20-$30.